This invention relates to light sources and more particularly to a light source capable of providing highly collimated light in at least one direction for use in increasing the brightness and color saturation of color liquid crystal displays.
It will be appreciated that color liquid crystal display panels are routinely utilized in laptop computers. However, these displays are not generally sunlight viewable due to the reflection of the sunlight back towards the viewer. In an effort to provide such displays with enough brilliance and enough color saturation to be able to be viewed in direct sunlight, recently a liquid crystal display has been provided with a microlens array in which the colors from a light source are separated out into distinct patches which illuminate the various red, green and blue sub-pixels associated with the display.
It is important in the microlens arrays that the diffraction gratings be illuminated by collimated light. The reason that this is important is that the surface structure of the diffraction grating provided by the microlens display is calculated utilizing a collimated light source. When the light source is not collimated, the image produced by the diffraction grating along with the fresnel focusing used by the microlens is defocused or smeared out. This means that for instance red light which is to be focused on a red sub-pixel in the liquid crystal display can in certain instances be smeared out sufficiently to overlap an adjacent green sub-pixel.
In order to assure that such smearing does not occur, it is important to provide a light source which is at least collimated in one direction with respect to the liquid crystal display.
In the past, displays having collimated light sources have been utilized to eliminate various pixel effects produced by a display. By pixel effects is meant off-angle, gray-scale changes and color inversion. It is, however, only with difficulty that a compact collimated light source can be provided.
It will be appreciated that collimated light sources as applied to liquid crystal displays are not common due to the difficulty of incorporating a collimated light source in a relatively thin display module. It will also be appreciated that semi-collimated light sources are utilized in projection liquid crystal display systems in which very bright light point sources are utilized. However, collimation must take place over an extended distance unsuitable for laptop computers and other applications where space is at a premium.
As is well known, collimated light can be achieved simply by removing the 360 degrees light source a large distance from the intended point of use. However, most of the light is lost in such an arrangement. In order to provide for more intense collimated light, reflectors are required which redirect the light available from the source and project it in a single direction. The length and configuration of such reflectors is unusable in portable display applications.
By way of further background collimation is taken to mean that the light from a source is emitted in a given direction and diverges by no more than some small angle. It is the purpose of a light collimator to assure that all of the light coming out of the source travels along a given direction. Typically when a slit source is utilized, the light from the slit exits in an arc are bounded by 180 degrees.
In the subject invention a lambertian light source in the form of a fluorescent tube is used with light exiting a slit redirected and collimated using an optical waveguide with parabolic surfaces, each having a focus at an opposing exit slit edge. In one embodiment, this collimated light is folded back on itself and is both expanded by a reflective array to provide an extended light source (a panel of light) and is further collimated by cylindrical lens array.
In one embodiment a diffuse reflector used around the fluorescent tube for maximum efficiency in condensing the light to a narrow slit, similar to an integrating sphere. The diffuse reflector is able to direct light from the fluorescent tube to a relatively narrow slit at an efficiency of 75%. The result is that a large amount of light is available at the exit slit.
However, the light available at the slit is not collimated. By utilizing an optical waveguide in which the surfaces are parabolic it is possible, in one instance, to collimate the 180 degree light into a beam of +/xe2x88x923 degrees along in the horizontal direction assuming the lamp and slit are oriented vertically. By providing top and bottom parabolic surfaces to the waveguide, it is possible to collimate light in the vertical direction to +/xe2x88x9210 degrees. The foci of these parabolas are set such that a parabola has its focus at an opposing edge of the exit slit.
In one embodiment, the collimated beam is folded back by two 45xc2x0 mirrors, with the light being redirected towards an entrance port of a reflective array in which the array has 45 degree reflectors spaced along its back edge. The purpose of the reflection array is to distribute the light across the extended area of an LCD panel. To accomplish this, the array of reflectors reflects the light, via total internal reflection, down the array at right angles where it exits a face of the array. The face of the array in one embodiment is provided with collimating lenses which are cylindrical, whereby the exiting light in the horizontal direction is collimated to a +/xe2x88x92xc2xd degree angle; with light exiting in the vertical direction collimated to +/xe2x88x9215 degrees.
It will be appreciated that having light collimated to a +/xe2x88x92xc2xd degree angle permits a microlens diffraction separator array to provide a high degree of color separation. Note, the collimation in the vertical direction is not as important in one embodiment due to a horizontal arrangement of the sub-pixels in the associated liquid crystal display in which the red, green, and blue sub-pixels are arrayed on a horizontal axis. Since the light from the subject collimator is highly collimated in the horizontal direction, color separation is adequate. On the other hand, the light being not so highly collimated in the vertical direction does not deleteriously affect the display. This is because the colored light will merely fall on a sub-pixel having the same color.
What has been described in one embodiment involves a 1 millimeter exit slit and a parabolic waveguide of approximately 6 inches in length. In a second embodiment, a much more compact collimator is provided by making the effective width of the exit slit on the order of a couple of microns in width. In this embodiment the extended waveguide is replaced with a thin waveguide sheet having a number of side-by-side internal parabolas formed therein. The entrance apertures for these parabolas are in the micron range to give an equivalent exit slit in the micron range. The portions of the sheet adjacent the apertures are provided with highly reflective material such that light not entering the very small apertures is reflected back towards the light source and then re-reflected or redirected by the reflector back towards the sheet. As a result the amount of space required to provide the collimated light is drastically reduced due to the provision of the waveguide sheet and the subminiature apertures involved. As before, the parabolas are configured such that the parabolic surface of one side of a parabola is focused at an edge of the opposing exit slit.
It will be appreciated that while the subject invention has been described in terms of parabolic shapes for the waveguide, other shapes which are called non-imaging collimator shapes can be substituted to provide for a more uniform intensity output across the aperture of the waveguide.
As will be discussed, the parabolas in one embodiment are generated through the utilization of a program which is written in FORTRAN modified with special calls by ASAP optical design code.
In one embodiment the waveguide is made of a clear acrylic, although any clear non-diffuse glass is substitutable therefor with appropriate modification in dimension due to index of refraction differences.
As mentioned, the subject collimator may be utilized in a liquid crystal display in which microlenses are used to focus regions of color towards associated sub-pixels in the liquid crystal display. One such arrangement is shown in U.S. Pat. No. 5,600,486, incorporated by reference, which describes a color separation microlens. This type of microlens array can, as illustrated in U.S. Pat. No. 5,781,257, incorporated by reference, be utilized in a flat panel display. In both cases the color separation depends upon collimated light which can be provided by a subject collimator.
Moreover, as illustrated in U.S. Pat. Nos. 5,482,800 and 5,310,623, both incorporated by reference, methods are described for fabricating the above mentioned microjenses. Finally, U.S. Pat. No. 5,497,269, incorporated by reference, shows a dispersive microlens used for detecting multiple, different wavelengths and for combining a plurality of different, emitted wavelengths.
In summary, a collimator is provided to collimate light which originally leaves an exit slit at 180 degrees and is collimated in one direction to a +/xe2x88x92xc2xd degree angle; and in an orthogonal direction at +/xe2x88x9215 degrees. The collimator is utilized in one application to maximize the diffraction efficiency of a diffractive color separator microlens array to increase the brightness of a color liquid crystal display and to increase color saturation. In order to provide collimation, the light exiting an exit slit of a lambertian light source in the form of a fluorescent lamp is focused by parabolic surfaces of an optical waveguide to provide a beam which has a collimation of +/xe2x88x923 degrees in one direction and +/xe2x88x9210 degrees in an orthogonal direction. In one embodiment, the beam is folded back on itself and is injected into a stepped reflective array which is provided with a stepped back surface that reflects the injected light and presents it across a broad panel. The panel is provided with cylindrical collimating lenses which further collimate the light in the original direction to a +/xe2x88x92xc2xd degree angle. Collimation in the orthogonal direction is +/xe2x88x9215 degrees in one embodiment. The folded nature of the collimated light source permits compactness with the collimated light distributed across a wide surface to provide a panel from which collimated light exits. The walls of the collimator are parabolic, with the parabola of a wall of the waveguide having its focus at the opposing edge of the exit slit. The collimation in the orthogonal direction is provided by providing the waveguide with parabolic top and bottom surfaces, with the parabolas of these surfaces having their foci at opposite top or bottom edges of the exit slit. For an extended collimated light source the folded collimated light sources can be stacked or arranged so as to provide a wide panel of collimated light, at least in one direction. For an ultra thin collimated source, a sheet containing side-by-side internal parabolas is substituted for the extended optical waveguide and is placed adjacent the exit slit of the lambertian light source, with the entrance apertures in the sheet having a width in the micron range.